The objective of this project is the development of an in vitro bioassay with drug metabolizing capacity to detect environmental teratogens. We have developed a system in which hamster embryo cells (HEC) are co-incubated with mouse embryonic limb buds which may serve as a screen for teratogens requiring mixed function oxidase (MFO) metabolism. HEC have been shown to activate the teratogen cyclophosphamide to metabolites which bind to limb bud DNA and induce abnormal limb development in vitro. We have found that HEC can be harvested and frozen at the first passage, and used for subsequent studies where MFO activity is induced by addition of phenobarbital to the media. Adult hepatocytes were cultured to determined if they could function as an activating system when co-incubated with limb buds. Media components essential for maintenance of adult hepatocytes, such as insulin and thyroxine, were found to be incompatible with normal development of limb buds in vitro. Studies are continuing in which a spectrum of environmental agents requiring MFO metabolism to exert toxicity will be tested in this system.